Since the early days of semiconductor device development, researchers have proposed metal base transistors, i.e., electronic devices in which electrons are injected from a semiconductor emitter into a thin metal base layer. For at least one type of metal base transistor, it is required that a significant fraction of the injected electrons traverse the base layer without being scattered, and furthermore, penetrate a metal/semiconductor interface to enter into the semiconductor collector.
Early theoretical treatment of such structures suggested that practical metal base transistors of this type were achievable without the necessity for further significant advances in technology. For instance, in a paper by J. L. Moll (IEEE Transactions on Electron Devices, Vol. ED/10, pp. 299-304, (1963)) it was stated that gold appears to be the most favorable metal for the metal base, that a 200 .ANG. thick gold film will transmit approximately 60 percent of the injected electrons, and that the quantum mechanical reflection at the base/collector interface is probably very small.
However, careful later work showed that, because of fundamental physical limitations, even the performance of proposed optimal metal base transistors would fall far short of previous expectations. For instance, the Si/Au/Ge metal base transistor was found to have a maximum low frequency common base current gain in the neighborhood of 0.3. It was also stated that it is unlikely that a materials combination exists that is capable of yielding substantially higher current gain. (S. M. Sze et al, Solid State Electronics, Vol. 9, pp. 761-769, (1966), incorporated herein by reference.) The authors of that paper also concluded that the greatest unsolved technological problem in semiconductor/metal/semiconductor transistor technology is the deposition of device-quality semiconductor materials on thin metal films, since, as of the date of the paper, there had been no reports of epitaxial growth of semiconductor material on films of the metals with longest electron mean free paths, namely, gold and silver.
Possibly to avoid the above-referred to technological problems, transistor structures that avoided the necessity to grow device-quality semiconductor material atop a metal layer were proposed and built. For instance, C. O. Bozler et al (IEEE Technical Digest, International Electron Device Meeting, pp. 384-387, (1979)), disclosed a permeable-base transistor in which an extremely fine tungsten grating is embedded inside a single crystal of n-type gallium arsenide, with the grating providing means for controlling the current from the emitter to the collector.
A major step towards realization of practical ballistic electron devices was the discovery, by J. C. Bean et al, of a method for preparing metal silicide/silicon heterostructures that yield a device-quality metal layer, the silicide layer, atop a single crystal silicon substrate. Since the silicide layer could be grown to be epitaxial with the substrate, the silicide layer could form the substrate for growth thereon of an epitaxial semiconductor layer, making possible the growth of epitaxial heterostructures incorporating a metal layer. See U.S. patent application Ser. No. 156,649, filed June 5, 1980, now U.S. Pat. No. 4,492,971, and U.S. patent application Ser. No. 445,014, filed Nov. 29, 1982, now U.S. Pat. No. 4,554,045 both co-assigned with this and incorporated herein by reference. In the former application are disclosed also articles comprising a single crystal silicon substrate, a single crystal metal silicide layer overlying the silicon substrate, and a single crystal silicon layer overlying the metal silicide, with the thus formed composite structure having an essentially continuous single crystal lattice between the three layers. Cobalt silicide and nickel silicide are exemplary silicides disclosed in the above applications.
Further relevant disclosures were made by Gibson et al (U.S. patent application Ser. No. 429,291, now U.S. Pat. No. 4,477,308, co-assigned with this), and incorporated herein by reference, namely, a method for growing epitaxial silicide on silicon comprising deposition of a thin template layer. This approach permits, inter alia, control of the orientation of silicide grown on (111)-oriented silicon, where the epitaxial material can grow in one of two orientations (A or B), or as a mixture thereof. H. Ishiwara et al, Materials Research Society Symposium Proceedings, Vol. 25, pp. 393-403 (1984) disclose growth of Si on a CaF.sub.2 substrate by pre-deposition of a thin Si layer at room temperature, followed by deposition thereon of a thick Si film at 800.degree. C.
Although great strides have recently been made in the growth of epitaxial metal layers on semiconductors, we know of no practical ballistic electronic devices. Because of the great promise of such a device, especially its potential for very high frequency operation, such a device would be of considerable commercial interest. This application discloses such a device.